Cables containing optical fibers are used to transmit information, including voice, video and data signals, over long distances. They can be grouped into three main categories, which are distinguished by the location of the optical fibers within the cable. For example, in loose tube fiber optic cables, the optical fibers lie in one or more buffer tubes that are stranded about an elongated central strength member. Each of the buffer tubes usually includes a water-blocking material, such as a gel or other filler material that prevents moisture intrusion. In cases where the buffer tube count is less than the maximum number that can be stranded about the central strength member, the loose tube designs may include one or more flexible filler rods. The filler rods, which are typically fabricated from solid or cellular polymers, are wrapped about the central strength member and help minimize gaps between the central strength member and an outer protective covering or polymer jacket.
Other fiber optic cable designs include monotube and slotted core cables. In monotube cables, the optical fibers are contained within a single, central buffer or core tube that contains a water-blocking agent. In slotted core cables, the optical fibers reside in channels or grooves that have been formed on a surface of a rod-shaped polymer core. The grooves typically follow a helical path along the surface of the core, which reduces compressive and tensile forces on the optical fibers whenever the cable is twisted, stretched, bent or compressed. The helical path traversed by the grooves may reverse direction at regular intervals along the cable's longitudinal axis, which further reduces the forces acting on the optical fibers. In addition to a central strength member and a water-blocking agent, which is disposed in each of the grooves, slotted core cables usually include a buffer tube that covers the slotted core. Both monotube and slotted core cables also include an outer protective covering or polymer jacket.
Each of the fiber optic cable designs—loose tube, monotube, slotted core—may include other components, including reinforcing yarns and fibers, rip cords, strength members and additional water-blocking materials (hot melts, water swellable powders, etc.). The fiber optic cables may also include helically wrapped tapes, corrugated armor and similar layers that help protect the optical fibers within the cable.
The buffer tube or core provides the primary protection for the optical fiber. As a result, the buffer tubes should be robust, exhibit good resistance to compressive, tensile and twisting forces (i.e., crush resistance) while maintaining adequate flexibility over a wide range of temperatures. Other desirable properties include low cost and low moisture sensitivity, as well as good heat resistance, dimensional stability (e.g., low coefficient of thermal expansion) and chemical resistance. A useful parameter for measuring or indicating the robustness, crush resistance and flexibility of a polymer is flexural modulus.
Currently optical fiber buffer tubes are manufactured using polypropylene (PP), polyethylene (PE), copolymers of PP and PE, nucleated copolymers of polypropylene and polyethylene (n-PP), polybutylene terephthalate (PBT), polyamides such as nylon 12, polycarbonate (PC), fluoropolymers, and multiple layers of these materials, such as a dual-wall tube using polybutylene terephthalate and polycarbonate (PBT/PC).
Though useful, none of these materials is completely satisfactory. PBT and PBT/PC are robust, exhibit good crush resistance and are perhaps the most widely used materials for buffer tubes, but they have marginal flexibility, exhibiting a flexural modulus in excess of about 370 kpsi at room temperature. Because of the high flexural modulus and robustness, PBT buffer tubes have a tendency to kink when they are bent instead of uniformly bending with a smooth, gradual radius. Though PBT can be treated to make it more flexible, such treatments increase its cost, making it less attractive for buffer tube applications. Additionally, PBT is susceptible to hydrolysis, which results in a loss of strength following exposure to moisture. Polyamides also are susceptible to hydrolysis and tend to be hygroscopic, which negatively impacts their mechanical and electrical properties and their dimensional stability.
Polyolefins such as PE, PP and copolymers of PE and PP each have a flexural modulus less than about 180 kpsi and therefore exhibit good flexibility. However, these materials generally possess poor crush resistance, making them less useful for buffer tube applications. As compared to PBT, polyolefins such as PE, PP and n-PP exhibit lower tensile, flexural and compressive strength, and lower thermal resistance. Furthermore, PE, PP and many of the copolymers of PE and PP undergo post-extrusion shrinkage, which may result in an increase in excess fiber length (ratio of optical fiber length to buffer tube length) over PBT. Increases in excess fiber length may lead to increased signal attenuation.